1. Field of the Invention
The present invention relates to a control device for starting a fuel cell vehicle, and in particular, relates to a technique for starting the fuel cell in a hybrid-type power source device provided with a power storage unit for assisting a power supply from the fuel cell.
2. Description of the Related Art
Conventionally, a hybrid-type fuel cell power generation system is known, which comprises a fuel cell and a power storage unit such as a battery or a capacitor (electric double layer capacitor or condenser) in order to compensate the output responsiveness of the fuel cell, which is driven by a fuel gas supply. The fuel cell vehicle is installed with a solid polymer-type fuel cell, which is composed of a plurality of cells, each of which is formed by sandwiching a solid polymer electrolyte membrane corresponding to a solid polymer ion exchange membrane between an anode and a cathode.
In the above conventional hybrid-type fuel cell power generation system, when starting the fuel cell, first, air is supplied to a pressure control valve at the fuel side, for example, and fuel gas is supplied to the fuel electrode in response to air pressure supplied to the pressure control valve.
Thus, before starting the fuel cell, the power storage unit supplies electric driving power to the compressor which supplies air. In addition to auxiliary devices for driving the fuel cell, the power storage unit supplies electric power to the motor for driving the vehicle when the vehicle starts immediately after the start of the fuel cell, so that the power stored in the power storage unit is reduced, and the voltage between both terminals of the power storage unit is reduced.
When the fuel cell is connected to the power storage unit, whose voltage between both terminals has been reduced, a large current rapidly flows from the fuel cell to the power storage unit. In the course of translating the voltage between both terminals of the fuel cell and the power storage unit into a balanced state, the voltage between both terminals of the fuel cell is reduced. Then, the fuel cell is in danger of losing hydrogen or the water content in the solid polymer electrolyte film, or of experiencing a decrease in the durability of the fuel cell.
As described above, if the fuel cell is connected to the capacitor under a condition that the terminal voltage of the capacitor greatly differs from the terminal voltage of the fuel cell (output voltage of the fuel cell) at the time, for example, of starting the fuel cell, a large current may flow, which may cause a danger that the performance or the durability of the fuel cell will become deteriorated.
In the conventional fuel cell power source device, when the fuel cell is being started, air is first supplied to the pressure control valve for supplying fuel, and fuel supply is started in response to the air pressure supplied to the pressure control valve.
Thus, prior to activation of the fuel cell, a driving electric power is supplied to the fuel supply device such as an air compressor. In addition to the activation of the fuel cell driving auxiliary machines and various control devices, electric power is supplied from the capacitor to the propulsion motor, which is driven immediately after starting the fuel cell vehicle, which results in reducing the capacitor energy causing depression of the terminal voltage of the capacitor.
If the capacitor, in which terminal voltage has been depressed, and the fuel cell are connected, a large current flows from the fuel cell towards the capacitor, the terminal voltage of the fuel cell is reduced in the course of recovering both terminal voltages of the fuel cell and the capacitor to an equilibrium voltage. When the terminal voltage of the fuel cell is depressed, the performance or the long-term stability of the fuel cell are in risk of being deteriorated, or hydrogen and water in the solid polymer electrolyte membrane are in danger of being evaporated.
As described above, when the fuel cell and the capacitor are connected to each other under conditions that the terminal voltage of the capacitor is far below the terminal voltage of the fuel cell, a large current flows from the fuel cell to the capacitor, which may cause a danger to the performance and long-term stability of the fuel cell will be deteriorated.
The present invention was made in order to solve the above problems and an object of the present invention is to provide a control device for starting the fuel cell vehicle and which is capable of preventing an excessive reduction of the voltage between both terminals of the fuel cell.
One aspect of the present invention provides a control apparatus for starting a fuel cell vehicle comprising: a fuel cell (for example, a fuel cell 11 in the embodiment) supplying electric power to a load (for example, driving motor 13, PDU 14, and air compressor 15, etc. in the embodiment); a capacitor (for example, power storage unit 12 in the embodiment) for assisting the supply of electric power to the load and for storing generated energy of the fuel cell; a fuel cell driving device (for example, air compressor 15 in the embodiment) for supplying reaction gases (for example, hydrogen gas and air in the embodiment) and for driving the fuel cell; and a current limiting device (for example, a secondary precharge circuit 17 in the embodiment) for limiting an output current (for example, output current Ifc in the embodiment) from the fuel cell; wherein, at the time of starting the fuel cell, the capacitor supplies electric energy to the fuel cell driving device and the current limiting device prohibits the fuel cell from outputting an output current until an output voltage (for example, output voltage Vfc in the embodiment) of the fuel cell reaches a predetermined voltage, and, after the output voltage rises to more than the predetermined voltage (for example, VMOT≈Vst≈Vfc in the embodiment), the current limiting device limits the output current of the fuel cell to below a predetermined current value until the difference between the output voltage of the fuel cell and a terminal voltage of the capacitor (for example, terminal voltage Vst in the embodiment) reaches a predetermined voltage difference (for example, predetermined voltage difference xcex94V in the embodiment).
According to the above constitution of the control apparatus for starting a fuel cell vehicle, since the output current of the fuel cell is limited at the time of starting the fuel cell, it is possible to prevent the terminal voltage of the fuel cell from reducing rapidly.
That is, at the time of starting the fuel cell, air is supplied to the pressure control device for supplying fuel to the fuel electrode of the fuel cell in addition to the air electrode of the fuel cell. In this case, the capacitor supplies electric power to the fuel cell driving device and the terminal voltage of the capacitor reduces. By providing a primary precharge circuit, which is provided with a resistor having a relatively high resistance, the capacitor can output a limited current to the fuel cell driving device or the power drive unit of the driving motor through the resistor, to thereby prevent the fuel cell from generating large output current.
In the above case, the DC-DC chopper prohibits the output current flowing from the fuel cell until the output voltage of the fuel cell reaches a predetermined voltage, and, the DC-DC chopper limits the output current flowing from the fuel cell, even after the output voltage of the fuel cell has been reached to a predetermined voltage, thereby, it is possible for a large current to flow from the fuel cell to the capacitor.
The above mechanism prevents a large current from flowing from the fuel cell to the capacitor until the voltage difference between the terminal voltage of the fuel cell and the terminal voltage of the capacitor become less than a predetermined value. Thus, in that period, the capacitor is gradually charged by a limited current from the fuel cell, and the terminal voltage of the capacitor approaches to the terminal voltage of the fuel cell. Therefore, it is possible to prevent evaporation of water or hydrogen from the solid polymer electrolyte membrane of the fuel cell and also to prevent the fuel cell from losing long-term stability, which contributes to maintain the performance and long-term stability.
By the use of the DC-DC chopper as a current limiting device, it becomes possible to control the output current of the fuel cell by changing a duty ratio of a pulse current which is input for controlling the chopping operation, and to reduce a time period until both terminal voltages of the fuel cell and the capacitor attain an equilibrium while preventing a rapid drop of the terminal voltage of the fuel cell.
The second aspect of the present invention provides a fuel cell power source system comprising: a fuel cell for supplying electric power to a load, an electric power storage device for assisting supply of electric power to the load, and a switching device (for example, a DC-DC chopper 17a in the embodiment), disposed between the fuel cell and the capacitor, for switching connection or disconnection of the fuel cell with the capacitor; and a control device (for example, a control portion 17b in the embodiment) for controlling the switching device, wherein when the fuel cell is being connected to the capacitor, the control device detects the voltage difference between a terminal voltage of the capacitor (for example, a terminal voltage of the capacitor Vst in the embodiment) and a terminal voltage of the fuel cell (for example, a terminal voltage of the fuel cell Vfc in the embodiment), when the voltage difference is larger than a predetermined value, the control device executes a chopping control of the switching device.
The third aspect of the present invention provides a fuel cell power source system comprising: a fuel cell for supplying electric power to a load, a capacitor for assisting supply of electric power to the load, a connecting device (for example, a DC-DC chopper 17a and a current limiting device 16b in the embodiment), disposed between an output end of the fuel cell and the capacitor, for connecting an output end of the fuel cell and the capacitor, and a control device (for example, a control device 17b and a fuel cell control device 32 in the embodiment) for controlling the connecting device as to whether the fuel cell is connected or disconnected with the capacitor, wherein when the fuel cell is being connected to the capacitor, the control device detects the voltage difference between the terminal voltage of the fuel cell (for example, a terminal voltage of the fuel cell Vfc in the embodiment) and the terminal voltage of the capacitor (for example, a terminal voltage of the capacitor Vst in the embodiment), and when the voltage difference is larger than a predetermined value, the control device controls the connecting device so as to limit an amount of a current (for example, an output current Ifc in the embodiment) flowing from the fuel cell to the capacitor.
The fourth aspect of the present invention provides the fuel cell power source system, wherein the control device connects the fuel to the capacitor after the fuel cell has been activated.
The fifth aspect of the present invention provides a fuel cell power source system which comprises a control device comprising a primary precharge circuit (for example, a primary precharge circuit 16 in the embodiment), disposed downstream of the capacitor, comprising a switching device (for example, a switching device 16a in the embodiment) and a current limiting device (for example, a current limiting device 16b in the embodiment), and a secondary precharge circuit (for example, a secondary precharge circuit 17 in the embodiment), disposed downstream of the fuel cell, comprising a chopping device (for example, a DC-DC chopping device 17a in the embodiment) and a chopper control device (for example, a chopper control device 17b in the embodiment), wherein when the voltage difference between the terminal voltage of the fuel cell and the terminal voltage of the capacitor exceeds a predetermined value, the current limiting device of the primary precharge circuit and the DC-DC chopper of the secondary precharge circuit control an amount of current flowing from the fuel cell flowing to the capacitor, and when a voltage difference between the terminal voltage of the fuel cell and the terminal voltage of the capacitor is reduced below the predetermined value, the primary precharge circuit and the secondary precharge circuit allow current flowing from the fuel cell to the capacitor and to the load.
As shown in FIG. 3, the control apparatus includes a DC-DC chopper 17a, in which ON/OFF of the transistor TR is controlled by supplying a pulse current to the base of the transistor. The control device 17b changes the duty ratio of the pulse current (the ratio of ON/OFF) so as to extend the OFF state of the transistor.
The sixth aspect of the present invention provides a method for controlling start of a fuel cell vehicle, the fuel cell having a fuel cell provided with a fuel cell driving device for supplying electric power to a load, an electric power storage device for assisting supply of electric power to the load, the control apparatus for controlling the fuel cell power source system having a primary precharge circuit disposed downstream of the capacitor, comprising a switching device and a current limiting device; and a secondary precharge circuit disposed downstream of the capacitor, comprising a DC-DC chopper and a chopper control device, the control method comprising the steps of opening the switching device of the primary precharge circuit when the terminal voltage of the capacitor and the terminal voltage of the load reach an equilibrium voltage after supplying a limited current from the capacitor through the current limiting device (for example, a current limiting device 16b in the embodiment), activating the fuel cell by activating the fuel cell driving device by supplying fuel to the fuel cell, detecting a voltage difference between the terminal voltage of the fuel cell and the terminal voltage of the capacitor, executing a chopping control of the output current of the fuel cell by the DC-DC chopper when the voltage difference exceeds a predetermined value when the voltage difference is reduced to be less than a predetermined value, and supplying the output current from the fuel cell to the load when the voltage difference is reduced below a predetermined value.
The seventh aspect of the present invention provides a method of controlling a fuel cell power source system, the fuel cell power source system having a fuel cell provided with a fuel cell driving device for supplying electric power to a load, a capacitor for assisting supply of electric power to a load including a driving motor, and a control apparatus for controlling the fuel cell power source system having a primary precharge circuit disposed downstream of the capacitor, comprising a switching device and a current limiting device; and a secondary precharge circuit disposed downstream of the capacitor, comprising a DC-DC chopper and a chopper control device, wherein the control method comprises the steps of detecting a voltage difference between a terminal voltage of the capacitor and a terminal voltage of the fuel cell, limiting the output current of the fuel cell by the DC-DC chopper of the secondary precharge circuit when the voltage difference exceeds a predetermined value, and opening the secondary precharge circuit to supply the output current from the fuel cell to the capacitor and to the load when the second voltage difference reduces to be less than a second predetermined difference.
The fuel cell control device 32 outputs a rotation number command value N as the driving order to the auxiliary devices and also controls the primary and secondary precharge circuits 16 and 17, and controls the contact points of relays provided at the high voltage switch 16a and current limiting device 16 in the primary precharge circuit 16 and also outputs the current command value IFCCMD as the switching command for the chopper 17 so as to make the DC-DC chopper in the secondary precharge circuit execute the chopping control of the output current. This chopping control of the output current controls the time period for the terminal voltages of both the capacitor and the fuel cell to reach an equilibrium. Moreover, the chopping control by the DC-DC chopper easily limits the current generated by the fuel cell by changing the duty ratio of the pulse current, so that it is possible to reduce the time period until both terminal voltages of the fuel cell and the capacitor reach an equilibrium voltage.